The present invention relates to a distributed feedback laser device (will be abbreviated to a DFB laser device) which is suitable for using as a light source in middle and long distances-optical transmission systems.
As well known in the art, the distributed feedback laser device is suitable for using as the light source in the middle and the long distances-optical transmission systems because such a laser device has high output characteristic, high output efficiency characteristic, and excellent single-modality of oscillation wavelength and is capable of performing satisfactory function in coupling with the optical fiber.
Recently, it is requested to provide a laser module which is small in size and low in price. In order to be applied into the laser module, it is requested to provide the DFB laser device which does not need any temperature control and a control unit for the control and which is excellent in temperature characteristic. It is especially important that the DFB laser device has the high output and the high efficiency characteristics under a high temperature (e.g. 85.degree. C.).
To satisfy such a request, a semiconductor laser device having improved temperature characteristic has developed by applying a multiple quantum well or a strained multiple quantum well structure to an active layer.
On the other hand, a spot-size conversion laser device has been recently developed. The spot-size conversion laser device provides high output characteristic because the device is reduced in an emission angle and is enhanced in a coupling efficiency with optical fiber.
A beam emission shape, which is important to couple the semiconductor laser device with the optical fiber, becomes a shape that an electrical field distribution shape on an emission surface (a near field pattern) is transformed by Fourier transform. Generally, as the near field pattern is enlarged in width, the emission angle is reduced.
The spot-size conversion laser device is, for example, disclosed in "1997 International Conference on Indium Phosphide and Related Material, Conference Proceedings, page 657". In the proceedings, the spot-size conversion laser device is a lateral active tapered type spot-size conversion laser device with the whole of a cavity resonator (will be abbreviated to a cavity) formed by an active region or an active domain. Such a laser device is regarded as a promising device because the device has an excellent temperature characteristic and a high yield.
The semiconductor laser device disclosed in the proceeding is characterized by a good point that the width of an optical waveguide which serves as an active layer is narrower at the portion nearer a beam emission surface. The semiconductor laser device of the type is capable of shortening a device length relatively because the optical waveguide is substantially formed by the active region, which results in a advantage in a yield. In addition, the laser device has another good point that it can be manufactured by the same manufacturing process as that of a conventional semiconductor laser device.
As for the DFB laser device excellent in single-modality of oscillation wavelength, it is also necessary to consider the means to enhance the coupling characteristic with the optical fiber by reducing the beam emission angle when the device is coupled with the optical fiber.
For example, the lateral active tapered type spot-size conversion laser device mentioned above has a structure that the active layer, which combines with the optical waveguide, is reduced in width in a direction parallel to the beam emission surface. Namely, the active layer is changed in width along a cavity direction. With this structure, the active layer (the optical waveguide) is changed in an equivalent refractive index in the cavity direction.
Therefore, it is difficult to control an oscillation wavelength and the coupling efficiency, if the above-mentioned active layer structure is only applied to the DFB laser device.